When fuel is injected in the intake stroke of an in-cylinder fuel injection spark ignition internal combustion engine during a cold start in the engine such that homogeneous combustion is performed, a three-way catalyst which purifies the exhaust gas is not activated, and hence hydrocarbon (HC) in the exhaust gas generated by combustion of the fuel is discharged without being oxidized.
JP2000-145510A, published by the Japan Patent Office in 2000, proposes that during a cold start of an in-cylinder fuel injection internal combustion engine, the fuel injection amount be determined so as to generate an air-fuel ratio that is slightly leaner than the stoichiometric air-fuel ratio, whereupon fuel is injected in the compression stroke.
When fuel is injected during the compression stroke, the injected fuel is less likely to become adhered to the cylinder wall surface than when fuel is injected during the intake stroke. Moreover, fuel injection during the compression stroke produces stratified combustion in the engine. As a result, less of the air-fuel mixture flows into the quench zone. Furthermore, the exhaust gas temperature rises, which accelerates the oxidation reaction of the HC in the expansion stroke of the engine. Hence the total amount of HC discharge decreases.